"When matter and antimatter meet, they annihilate each other and the mass is converted into energy--specifically, into gamma-rays. If a distant galaxy were made of antimatter, it would constantly be producing gamma-rays as it encountered the matter in the intergalactic gas clouds that exist throughout galaxy clusters.
"We do not see any steady stream of gamma-rays coming from any source in the sky. Therefore, astronomers conclude that there are not occasional 'rogue' galaxies made of antimatter. If there is any large amount of antimatter in the universe, it must encompass at least an entire galaxy cluster, and probably a supercluster. Once might postulate the existence of such antimatter superclusters, but then one would be faced with the problem of coming up with a mechanism that, shortly after the big bang, would have separated these now-gigantic clumps of antimatter from the neighboring clumps of mater. No such mechanism has yet been envisioned."
>Scott Dodelson is a scientist in the Theoretical Astrophysics Group at Fermi National Accelerator Laboratory. He offers a more detailed reply:
"The question of whether or not there is anti-matter in the universe has been around ever since the prediction of the existence of the anti-proton early this century. For reasons that I'll explain, most physicists don't believe there is much anti-matter around. But the fact that the question is still being asked (by many scientists) indicates that it has not been definitively answered. We may all be in for a big surprise!
"With that background, here is an overview of our present thinking. A simple way to test and see if there is anti-matter around is to send out a 'detector.' In this case, it is completely trivial to make a detector: it simply has to be made of matter! Any time matter collides with anti-matter, the two annihilate and produce lots of gamma rays. We have sent spacecraft to Jupiter and other planets. These objects didn't annihilate, so we know that our solar system does not contain much anti-matter.
"In fact, we can make a much stronger statement about the abundance of anti-matter by searching for gamma rays from other galaxies and clusters of galaxies. A typical cluster does not emit many gamma rays, so all the galaxies in it must be made solely of matter. It is possible that all the galaxies in a given cluster are made of matter while all those in another are made solely of anti-matter. If this were true, then there would be immense gamma radiation coming from the boundary regions between clusters of different types. At present, such radiation is not observed, a fact that again argues against this separation. Matter and anti-matter therefore must be separated on scales larger than cluster sizes (roughly ten million light years).
"There is a strong argument against the possibility that matter and anti-matter exist in equal numbers in our universe but are for some reason separated. This argument goes back to the early universe and asks, When must the matter and anti-matter have been separated? It must have been very early, when the temperature of the universe was roughly 500 billion Kelvins. If they hadn't separated by then, matter and anti-matter would have mutually annihilated, because the universe was very dense. Is it possible to think of a mechanism that separated matter from anti-matter when the universe was very hot and dense? Apparently not, for any way of separating them has to obey causality. Early in the history of the universe, when annihilation between matter and anti-matter was occurring, the farthest possible distances that were in causal contact with each other were about 100 kilometers. This size is a billion times smaller than the regions that would grow to be clusters. So it seems impossible that matter was separated from anti-matter on scales the size of clusters today. The most natural explanation is that the universe is made up only of matter and contains no large reservoir of anti-matter. In fact, there are theories which explain how such an asymmetry could have occurred.
"Having said all this, I want to reiterate that this is not the final word. The arguments I have presented are suggestive but not compelling. For this reason, some physicists are excited about the Alpha Magnetic Spectrometer, a device that the National Aeronautics and Space Administration wants to fly in the Earth's orbit that will look directly for anti-matter."